Thermoplastic starch is produced by mixing native starch with a plasticizer at a temperature above the starch gelatinization temperature, typically in the 70-90° C. range. This operation weakens the hydrogen-bonds present in the native starch leading to a fully amorphous free-flowing material. The resulting material is known as plasticized starch, destructured starch or thermoplastic starch (TPS). The properties and rheology of thermoplastic starch have been thoroughly investigated (Aichholzer 1998; Vergnes 1987; Villar 1995; Willett 1995). As such, the TPS is not a suitable material for most common uses. It is very hygroscopic and its properties and dimensional stability are strongly affected by the humidity level since water is a plasticizer for TPS. Also in presence of humidity, the amorphous TPS tends to reform its hydrogen-bonds leading to recrystallization (also called retrogradation) and in turn to embrittlement of the material.
This strong property dependence on plasticizer content can become an advantage however when the TPS is blended with another hydrophobic polymer. In this case, the hydrophobic polymer can protect the TPS from direct water contact and moisture uptake while the plasticizer level in the TPS can be used to tune the mechanical properties of the TPS. Therefore, the vast majority of work involving the use of starch as a material has focused on blending of TPS and other synthetic polymers (Averous 2004; Schwach 2004; Wang 2003). The synthetic material can be biodegradable to produce a fully compostable material or can be non-biodegradable to produce materials for longer-term applications. Examples of biodegradable blends include blends of TPS with polycaprolactone or polybutylene succinate which are two petrochemical based polymers. With the recent commercial introduction of poly(lactic acid) PLA, there has also been a high interest for PLA/TPS blends and these have been investigated in terms of their compatibility (Huneault 2007) and of their processing into injection molded product, biaxially oriented films (Chapleau 2007) and low density foams (Mihai 2007).
The compounding process used for the preparation of TPS/polymer blends is relatively complex. It must nominally enable the precise metering of starch and plasticizers, the starch gelatinization and the mixing of the TPS with the second polymer phase to obtain finely dispersed or finely segregated blend morphology. Additionally, more elaborate functions may be performed. For example, venting or devolatilization may be used to control volatiles levels. Interface modification through in situ interfacial reaction may be used to compatibilize the blend or to modify the starch interface. Surprisingly, very little scientific publications have focused on the effect of the process on the final blend properties and little guidance can be found as to what could be the best practices in terms of TPS/polymer blend compounding technology.
In order to prepare finely dispersed blends of thermoplastic starch and synthetic polymers, it is beneficial to prepare the blends using a sequence of operation carried out along a twin-screw extrusion process (Favis 2003; Favis 2005; Favis 2008; Rodriguez-Gonzalez 2003). In the process described in the Favis et al. patents (Favis 2003; Favis 2005; Favis 2008; Rodriguez-Gonzalez 2003), the basic ingredients for the making of thermoplastic starch, starch, water and glycerol, are first mixed in 50:25:25 proportions to form a suspension (also referenced as a slurry). This suspension is pumped into the extruder. Under the action of shear and heat, the starch and plasticizers (water and glycerol) are transformed into thermoplastic starch (TPS) through a well known transformation called “gelatinization”. Further along the extrusion process, the water is removed to get a water-free TPS that is solely plasticized by glycerol. Then, further along the process, a synthetic polymer is added and mixed with the TPS to form the TPS/polymer mixture that is the end result of the process. In the Favis et al. process, it is specified that the synthetic polymer must be added in molten form to prepare blends with an acceptable dispersion.
There are at least two problems with the process presented in the prior art described above. First the use of a suspension forces the use of a high initial water content because starch suspensions necessitate at least 50% liquid to be pumped into the extruder. Since a water-free TPS is desired, this involves a very high rate of devolatilization and in turn a lower production rate and higher energy need. According to the Favis et al. patents cited above however, this water is necessary to achieve proper TPS gelatinization. The second problem with the Favis et al. process is that the synthetic polymer must be fed to the twin-screw process in liquid form. Thus a single-screw extruder must be used to heat, melt and pump the polymer into the extruder. Favis 2005 and Favis 2008 teach that feeding in liquid (molten) form is necessary for the formation of finely dispersed blends and in turn to good retention of material ductility. Feeding a polymer in molten form requires an auxiliary unit such as a single-screw extruder that is able to heat and pump the polymer at high pressure and thus requires additional energy, involves additional cost in comparison to feeding the polymer at room temperature.
In another report (Seidenstucker 1999), thermoplastic poly(ester-urethanes) (TPU) were compounded with destructurized starch in a twin-screw extruder. This report describes two-step processes similar to Favis et al. in which thermoplastic starch (TPS) is first made by pre-mixing starch with a polyfunctional alcohol before introduction into the twin-screw extruder. This report also describes a single-step process in which starch is introduced into the twin-screw extruder followed by introduction of glycerol downstream to form the thermoplastic starch in the extruder, and then followed by introduction of TPU further downstream in the extruder. This report indicates that the throughput of the single-step process is reduced to one-third of the two-step processes. Only two-step processes are actually used to produce TPS/polymer blends in this report, and there is no description of how much or even whether water can be used in conjunction with the glycerol for forming the thermoplastic starch in the single-step process.
In yet another report (Wiedman 1991), a twin-screw extrusion sequence is described for a food processing extrusion line involving thermoplastic starch. In this case steam injection and an unspecified liquid feed are used. There is no description of any particular plasticizer composition involving polyfunctional alcohols and water and no description of any control over the ratio of water to polyfunctional alcohol in the plasticizer. Even if the unspecified liquid feed did contain polyfunctional alcohol, controlling the water:polyfunctional alcohol ratio would be very difficult using the steam injection process. Further, to introduce polymer into the line, either a feeder located at the same point as the starch feeder or a downstream twin-screw side feeder could be used. If the feeder is used to introduce dry polymer, the polymer would be added at the same point in the line as the starch. If the downstream twin-screw side feeder is used, the polymer would be introduced in liquid form.
There remains a need in the art for an efficient process of making thermoplastic starch/polymer blends.